The present invention relates generally to pneumatic-hydraulically operated tools and more particularly to such tools which are designed for use in installation of pull type fasteners.
In pneumatic-hydraulic installation tools of the same type as the present invention, a source of compressed air is normally selectively applied to opposite sides of a pneumatic piston by an operator controlled valve assembly so as to reciprocably drive the piston through driving and return strokes. This pneumatic piston is in turn connected to a master hydraulic piston reciprocably disposed within a cylinder forming a part of a closed hydraulic circuit and operates to reciprocate the hydraulic piston in unison therewith. The hydraulic piston in turn operates to transmit hydraulic pressure and fluid to a slave or driving piston reciprocably in a separate cylinder also forming a part of the closed hydraulic circuit which piston has apparatus associated therewith for setting a fastener. The hydraulic pulling and master pistons may be viewed as separating the hydraulic circuit into a driving portion and a return portion which are alternatingly pressurized in response to reciprocal actuation of the pneumatic piston. Accordingly, during operation of the installation tool the various hydraulic seals required therein are continuously subjected to an extreme pressure range extending from a low pressure which may be below atmospheric pressure to a high pressure well in excess of atmospheric pressure. This continual cycling of pressure on the seals and particularly the change between positive and negative pressure relative to atmospheric pressure has been found to be detrimental to seal efficiency and may result in premature failure or leakage thereof both in terms of loss hydraulic fluid and entry of air into the hydraulic circuit. Entry of air into a hydraulic circuit is extremely detrimental to operating efficiency of the circuit because of the relative volumetric changes which result from pressure changes applied thereto. Accordingly, the presence of air in a hydraulic system results in lost motion within the system.
Additionally, in some such tools, a seal failure on either the hydraulic master or pulling pistons may result in generation of a relatively high internal pressure which may cause damage to the tool or result in failure of additional seals necessitating repairs more extensive than might otherwise be required.
The present invention, however, provides a hydraulic reservoir which includes apparatus to maintain a continuous positive pressure on the hydraulic circuit during operation of the tool whereby the detrimental effect of cyclical positive and negative pressures on the seals may be avoided. The reservoir system is designed so as to establish a minimum positive pressure of a magnitude relatively low as compared to the operating pressures of the tool and also operates to provide a supply of make up hydraulic fluid to compensate to a limited extent for leakage during operation of the tool. Apparatus is also provided whereby the reservoir may be easily and conveniently depressurized such as may be desired during service of the tool. Additional apparatus is disclosed which enables the reservoir to be easily and conveniently refilled.
While the reservoir is directly connected to the return portion of the hydraulic circuit, means are also incorporated therein whereby the driving portion of the hydraulic circuit may be similarly subjected to a minimum positive pressure during operation of the tool.
The hydraulic circuit of the present invention also incorporates pressure relief means which will operate to relieve internal pressures which may be generated due to a piston seal failure on either the hydraulic master or pulling pistons thereby minimizing the possibility of resultant damage to the tool.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.